The effect of fluoridated milk on plaque bacteria was investigated in fourteen complete denture wearers. These subjects were randomly split into two groups, seven in the treatment only group and seven in the treatment + dentifrice rinse group. There were five treatment regimes, each of six-week duration. The subject either consumed milk once or three times a day; milk with fluoride once or three times a day; or no beverage consumption as a control. The purpose of the additional dentifrice rinse was to mimic the effects of brushing twice daily with a fluoridated toothpaste. Six enamel slabs were embedded at strategic locations on the upper denture to investigate the influence of salivary flow rate on aciduric bacteria within dental plaque. Plaque samples were obtained from each enamel slab at Weeks 1 and 6 of the experimental treatments, and subjected to microbiological analysis. Enumeration of the bacteria was carried out to obtain the total counts of plaque bacteria, the counts of S. mutans on selective MSB agar and counts of Lactobacillus spp. on selective Rogosa agar.l Statistical analysis revealed a main effect of treatment within the treatment only group for the lactobacillus counts. No beverage consumption and milk with fluoride consumed three times a day produced the lowest counts. Proportions of lactobacilli within the overall plaque bacteria were compared at the two buccal sites and one lingual site. The proportions at the buccal sites with a good salivary flow rate were lower than those that were experienced at the lingual site where there is a poor salivary flow. To investigate strain variation among clinical S. mutans isolates identified from unrelated individuals, ribotyping was employed. Analysis revealed that carriage of S. mutans genotypes was stable within an individual but that strain variability was diverse when analysed across all individuals. Longitudinal investigation revealed the possibility that S. mutans genotypes could be lost and then reappear at the same tooth site.